1. Field of the Invention
The invention relates to the security of telephone communications which utilize a composite channel telephone communications link, and in particular, to an apparatus and method to provide selectable voice security, by scrambling or encryption, of telephone calls over individual channels even though the calls travel in part over a composite communications telephone link such as T1 or E1 composite channel formats and in part over non-composite telephone communications links, including land line and cellular telephone networks, to the end point telephones.
2. Problems in the Art
Telecommunication transmission links can be either a loop, which connects a user with a serving office, or a trunk, which connects two offices. Telephone transmissions can be at voice frequency, or a number of voice-frequency channels can be multiplexed together using frequency-division techniques (analog carrier) or time-division techniques (digital carrier). The multiplexed signal can then be transmitted over guided wave media, such as wire and optical fibers, or through free space, as in radio or cellular systems.
The advantage of a multiplexed transmission is the greater economy of carrying many channels on a single medium element. One standardized digital carrier system or format is a 24-voice composite channel 1.544 Mb/s system referred to as T1. Voice channels (from a step-by-step switch, for example) are converted to digital form and multiplexed into digital groups in a primary pulse code modulation (PCM) multiplex also called a digital channel bank. This communications link can have terminals in, for example, public telephone company offices or be used at or between private company end-points.
The voice signals are converted to digital format by sampling and encoding the voice into digital words (e.g. 8 bit PCM). Each digitized voice or call is then multiplexed in synchronous fashion. Framing and signal bits are used to allow later de-multiplexing. From these individual frames, delineated by the framing bit, super frames of 12 and 24 frames are created. These super frames are used to encode additional call information. Each frame would also include all of the digitized voice for each of the 24 channels, (e.g. one frame contains twenty-four of the eight bit words which are the digital samples of the analog voice) as well as include one framing bit.
The digital information can also be encrypted if desired. It is not uncommon to do so over T1 lines. However, the digital information is usually bulk encrypted. Therefore, the security added to the communications link is without regard to the individual channels or calls. There can be no selective encryption of a call. They are all encrypted or none are encrypted.
Furthermore, such security, if even used, only exists between terminals of the T1 communications link. Ultimately, each call usually must traverse a T1 terminal and then another communications link before arriving at the end point receiving telephone set. Thus, there is generally no security level in that other link and any security in the T1 link is not helpful.
This security problem is well known. It is particularly significant because of the ever-increasing use of cellular telephones as the end point receiver. Any encryption or security in the T1 link is lost after individual calls in the link leave the T1 link. Cellular calls ultimately are transmitted in wireless fashion where anyone can intercept and listen, if they have the appropriate equipment and certain knowledge.
Some prior art has attempted to address communications security for cellular phones or radio links where the communications could be intercepted from the air. For example, U.S. Pat. No. 4,973,479 to Tobias, Jr. et al., teaches how a cellular phone with a scrambler or encryption mode could place calls to a phone without such capabilities. It uses a call diverter at, for example, a mobile telephone switching office (MTSO) to be instructed to route the scrambled or encrypted call through a scrambler/descrambler (or encrypter/decrypter). However, this does not address the problem of the security gap caused by lack of voice security after the terminal of a T1 communications link. It also requires a caller to proactively engage and instruct the diverter. U.S. Pat. No. 5,365,590 to Brame discloses a similar call diverter arrangement for radio communications.
Therefore, there is no known satisfactory solution to security problems regarding telephone calls that travel in part through a composite telephone link but in part outside the link. Furthermore, there is no known satisfactory solution to whether individual calls or channels on a composite channel can selectively be secured or unsecured.
It is therefore a primary object of the present invention to provide an apparatus and method for voice security for calls that are communicated in part through a composite digital telephone link, which improve over and solve the problems and deficiencies in the art.
Further objects, features, and advantages of the present invention include an apparatus and method as above-described which:
1. can provide scrambling or encryption security for voice telephone calls outside of the composite communications link. PA1 2. provide flexibility and selectability as to the security level desired for each channel of the composite communications link. PA1 3. can be transparent to the callers. PA1 4. do not require diversion steps. PA1 5. are economical. PA1 6. can be used by individuals, companies, or telephone companies. PA1 7. are non-complex. PA1 8. provide centralized security. PA1 9. allow secure calls to be initiated between devices equipped with equal or lower levels of security. PA1 10. allow various levels of security while providing good recovered voice quality.
These and other objects, features and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.